docs: def __make_footprint(input, size, footprint):
      "Creates a standard footprint element ala scipy.ndimage."
      if footprint is None:
          if size is None:
              raise RuntimeError("no footprint or filter size provided")
          sizes = _ni_support._normalize_sequence(size, input.ndim)
          footprint = numpy.ones(sizes, dtype=bool)
      else:
          footprint = numpy.asarray(footprint, dtype=bool)
      return footprint